In systems, such as telecommunications systems, it is often necessary to exchange circuit packs or cards while the remaining circuit packs or cards are in an operating state. When inserting a circuit pack there is no mechanical way to control how many backplane power pins make initial contact. For a 5 volt supply and a circuit pack requiring 60 Watts of power, an estimated instantaneous current can reach well over 75 A. If only a single power pin makes contact for as short a period as one millisecond, the pin would be damaged. A further concern is that the loading/unloading of the power supply, by insertion/withdrawal of the circuit pack, may exceed the regulating capability of the supply, thus effecting the operation of other circuit packs.
In U.S. Pat. No. 5,010,293, issued Apr. 23, 1991, William F. Ellersick teaches an inrush current limiting circuit for use in telecommunication systems. The problem solved is limiting inrush current when a -48 V to +5 V DC/DC power converter is plugged into a shelf powered by a -48 V power distribution bus. The circuit taught is not suitable for a +5 V power distribution bus.